Remove stray backtick in docs target

add GPU_CopyImageFromSurfaceRect

Fix include order so local paths are preferred to avoid compilation error.

Add camera demo

This function is like GPU_CopyImageFromSurface, but enable to copy only part
of the surface to the texture, using an optional pointer to GPU_Rect as
a new argument.

I've also reindent some code

Signed-off-by: matthias <uso.cosmo.ray@gmail.com>

fix issue #213

The problem seems to occur when sdl-gpu is installed, then CMakeList
was calling include_directories cmake function with SDL path before adding
project include/, So /usr/include/SDL had an higher priority than
sdl-gpu includes, and so it was using old versions,
which didn't have default_textured_vertex_shader_id.

Signed-off-by: matthias <uso.cosmo.ray@gmail.com>

Fix build on emscripten

* Use -s USE_SDL=2, do not try to find_package(SDL2 REQUIRED)
* Disable DEFAULT_DISABLE_GLES_1 which is not supported

Let SDL_gpu repo be easily added as a dependency in a CMake project

Store default shader ids

So they can be reused in other shader programs.

Guard to avoid double defining _USE_MATH_DEFINES

Fix warning in MSVC2019

Fix a warning about M_PI already being defined

Fix warning about casting intptr_t to void*