Add GLES support for the same pixel formats for copying texture -> buffer as when copying buffer -> texture (#183428)

`BlitCopyBufferToTextureCommandGLES` (and initializing/setting texture
contents in `TextureGLES`) supports a bunch of different pixel formats,
as defined in the `TexImage2DData` constructors in
`blit_command_gles.cc` and `texture_gles.cc`.

But `BlitCopyTextureToBufferCommandGLES` only has support specifically
for `kR8G8B8A8UNormInt` and `kB8G8R8A8UNormInt`.

This makes `BlitCopyTextureToBufferCommandGLES` support the same pixel
formats as `BlitCopyBufferToTextureCommandGLES` and `TextureGLES`.

Fixes https://github.com/flutter/flutter/issues/183462

## Changes:

### Share `ToPixelFormatGLES`

`blit_command_gles.cc` and `texture_gles.cc` have almost-duplicate logic
to convert `impeller::PixelFormat` to a `GLint internal_format`, a
`GLenum external_format`, and a `GLenum type`. Factor this out into a
shared `ToPixelFormatGLES` function in `formats_gles.h/cc`.

Where there were differences between the `blit_command_gles.cc` and
`texture_gles.cc`:
- `kR32G32B32A32Float`:
- `blit_command_gles.cc` has `internal_format = GL_RGBA`,
`external_format = GL_RGBA`
- `texture_gles.cc` has `internal_format = GL_RGBA32F`, `external_format
= GL_RGBA`
  - I used `internal_format = GL_RGBA32F`, `external_format = GL_RGBA`
- `kR16G16B16A16Float`:
- `blit_command_gles.cc` has `internal_format = GL_RGBA`,
`external_format = GL_RGBA`
- `texture_gles.cc` has `internal_format = GL_RGBA16F`, `external_format
= GL_RGBA`
  - I used `internal_format = GL_RGBA16F`, `external_format = GL_RGBA`
- `kR32Float`:
- `blit_command_gles.cc` has `internal_format = GL_RED`,
`external_format = GL_RED`
- `texture_gles.cc` has `internal_format = GL_R32F`, `external_format =
GL_RGBA`
  - I used `internal_format = GL_R32F`, `external_format = GL_RED`

All the other PixelFormat cases were the same between the two files and
copied over directly.

### Remove `TexImage2DData`

`blit_command_gles.cc` and `texture_gles.cc` each define a
`TexImage2DData` struct which just contains the pixel format data and
sometimes an optional data buffer. I removed both of these. Instead, we
just use the `ToPixelFormatGLES` return value directly, and use the
source data buffer directly.

### Improve pixel format support in
`BlitCopyTextureToBufferCommandGLES::Encode()`

In `BlitCopyTextureToBufferCommandGLES::Encode()`, use
`ToPixelFormatGLES()` to determine the format and type used for
`gl.ReadPixels()`. This expands the existing functionality that only
supported two specific pixel formats.

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we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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